.\"	$OpenBSD: pipe.2,v 1.18 2014/12/10 19:30:22 schwarze Exp $
.\"	$NetBSD: pipe.2,v 1.6 1995/02/27 12:35:27 cgd Exp $
.\"
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.\"     @(#)pipe.2	8.1 (Berkeley) 6/4/93
.\"
.Dd $Mdocdate: December 10 2014 $
.Dt PIPE 2
.Os
.Sh NAME
.Nm pipe ,
.Nm pipe2
.Nd create descriptor pair for interprocess communication
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn pipe "int fildes[2]"
.In fcntl.h
.In unistd.h
.Ft int
.Fn pipe2 "int fildes[2]" "int flags"
.Sh DESCRIPTION
The
.Fn pipe
function creates a
.Em pipe ,
which is an object allowing unidirectional data flow,
and allocates a pair of file descriptors.
The first descriptor connects to the
.Em read end
of the pipe,
and the second connects to the
.Em write end ,
so that data written to
.Fa fildes[1]
appears on (i.e., can be read from)
.Fa fildes[0] .
This allows the output of one program to be sent to another program:
the source's standard output is set up to be the write end of the pipe,
and the sink's standard input is set up to be the read end of the pipe.
The pipe itself persists until all its associated descriptors are closed.
.Pp
A pipe whose read or write end has been closed is considered
.Em widowed .
Writing on such a pipe causes the writing process to receive a
.Dv SIGPIPE
signal.
Widowing a pipe is the only way to deliver end-of-file to a reader:
after the reader consumes any buffered data, reading a widowed pipe
returns a zero count.
.Pp
The
.Fn pipe2
function is identical to
.Fn pipe
except that the non-blocking I/O mode on both ends of the pipe is
determined by the
.Dv O_NONBLOCK
flag in the
.Fa flags
argument and the close-on-exec flag on both the new file descriptors
is determined by the
.Dv O_CLOEXEC
flag in the
.Fa flags
argument.
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
.Fn pipe
and
.Fn pipe2
will succeed unless:
.Bl -tag -width Er
.It Bq Er EMFILE
Too many descriptors are active.
.It Bq Er ENFILE
The system file table is full.
.It Bq Er EFAULT
The
.Fa fildes
buffer is in an invalid area of the process's address space.
.El
.Pp
In addition,
.Fn pipe2
may return the following error:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa flags
is invalid.
.El
.Sh SEE ALSO
.Xr sh 1 ,
.Xr fork 2 ,
.Xr read 2 ,
.Xr socketpair 2 ,
.Xr write 2
.Sh STANDARDS
The
.Fn pipe
function conforms to
.St -p1003.1-2008 .
The
.Fn pipe2
function is expected to conform to a future revision of that standard.
.Pp
As an extension, the pipe provided is actually capable of moving
data bidirectionally.
This is compatible with SVR4.
However, this is non-POSIX behaviour which should not be relied on,
for reasons of portability.
.Sh HISTORY
A
.Fn pipe
function call appeared in
.At v3 .
Since
.At v4 ,
it allocates two distinct file descriptors.
The
.Fn pipe2
function appeared in
.Ox 5.7 .
